Investments - Amortized Cost and Fair Values of Debt Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Amortized cost
Due in one year or less	$ 667,446
Due after one year through five years	1,543,417
Due after five years through ten years	1,805,365
Due after ten years	1,003,052
Total debt securities with single maturity date, amortized cost	5,019,280
Amortized Cost	6,130,009	$ 5,926,785
Fair Value
Due in one year or less	659,512
Due after one year through five years	1,458,444
Due after five years through ten years	1,617,227
Due after ten years	800,405
Total debt securities with single maturity date, fair value	4,535,588
Total at end of period	5,581,537	5,409,698
ABS
Amortized cost
Total debt securities without single maturity date, amortized cost	172,262
Amortized Cost	172,262	126,723
Fair Value
Total debt securities without single maturity date, fair value	167,220
Total at end of period	167,220	120,687
RMBS
Amortized cost
Total debt securities without single maturity date, amortized cost	307,402
Amortized Cost	307,402	223,743
Fair Value
Total debt securities without single maturity date, fair value	272,682
Total at end of period	272,682	198,009
CMBS
Amortized cost
Total debt securities without single maturity date, amortized cost	297,091
Amortized Cost	297,091	257,785
Fair Value
Total debt securities without single maturity date, fair value	274,320
Total at end of period	274,320	237,216
CLOs
Amortized cost
Total debt securities without single maturity date, amortized cost	333,974
Amortized Cost	333,974	337,656
Fair Value
Total debt securities without single maturity date, fair value	331,727
Total at end of period	$ 331,727	$ 329,832